This invention relates to cooling apparatus for gas turbine blades, and more particularly to such apparatus utilizing a liquid coolant.
As is well-known in the art, one of the most effective methods for increasing efficiency of gas turbines is to elevate the inlet temperature of the motive fluid to the turbines. However, allowable temperature of metallic material used for turbine blades and the like is, generally, around 800.degree. C. Accordingly, employment of motive fluid with temperatures higher than such value without overheating the metal constituents requires that the members forming the turbines be cooled effectively and particularly that the blades be properly cooled.
Methods for cooling blades are divided roughly into air-cooling and liquid-cooling in which water is usually used as the coolant. Water is a superior coolant to air in general for two reasons. First, water has a higher thermal conductivity, and second, water can absorb more heat per unit mass due to its larger specific heat and to the available water-steam phase change. Thus, various ways of water-cooling turbine blades have been developed.
In such liquid-cooled rotating turbine blades, coolant passages beneath the blade surface travel in the longitudinal direction of the blades. The blades have a generally twisted configuration so that the coolant passages are generally not straight but also twisted in some extent. For purposes of illustration, however, the passages are shown herein as straight.
It is noted that coolant flow within such passages is subject to strong centrifugal force and also may be subject to Coriolis force. These conditions stratify the coolant flow such that the liquid travels as a thin film on the cooling passage wall, if the passage is not filled with liquid. The water-steam mixture within the passage flows in the form of film on the passage wall. This film flow tends to flow only on a portion of the passage wall so that such portion of the passage wall is more cooled than other portions of the wall on which no film exists. Non-uniform cooling causes relatively large thermal stress in the material so that the blades may suffer breakage.
One attempt to reduce the amount of thermal stress is disclosed in the U.S. Pat. No. 4,156,582. The coolant passages in this patent are provided by using preformed tubes located beneath an outer protective layer, and this layer is composed of an inner skin of high thermal conductivity and an outer skin for protection from hot corrosion. This approach to mollify local thermal stress suffers from difficulty and expense in manufacturing.
Another attempt to overcome these problems is feeding water to flow in the passage in full channel whereby the water contacts all of the passage wall. For example, U.S. Pat. No. 3,902,819 discloses the technique wherein the water flowing through the coolant passages is maintained at a super-critical pressure so that it cannot vaporize. However, this reduces substantially the amount of heat that can be absorbed because there is no utilization of heat absorption due to water-steam phase change. Further this approach requires that water fed in the cooling passages is introduced at the supercritical pressure.
Generally, water-steam mixture which has absorbed heat from the blades is drained into the flow of motive fluid from the cooling system of the blades. Draining of water-steam mixture is likely to cause impact erosion of the blades themselves or other parts including stationary parts of the turbine.